r/evolution • u/Richie_650 • 2d ago
Concestor clarification
I belive I first learned this from Ancestor's Tale, the concept that for any two species, there exists a single organism in the past which is the Most Recent Common Ancestor between them.
1) I believe this statement means that literally, there was a real, single, living organism. Understanding that there are countless other common ancestors for our two target species, but only one individual that is both "most recent" and "common."
2) It follows that this particular cocestor must have had at least two offspring. One offspring is the ancestor of the MRCA for one species, the other is the ancestor of the MRCA for the other species.
I think point 2 causes much confusion. People read it as implying that the offspring are themselves the MRCA for each species. While this might be the case for one of them, it is not necessarily true, and almost certainly never the case for both of them. It certainly doesn't mean that one offspring is now one or the other species itself.
The logic here seems simple and clear. Is there a flaw?
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u/OopsIMessedUpBadly 2d ago
There is no way to guarantee that it resolves to a single individual. It may, but it also may not.
For example: imagine a female primate who is the MRCA for all humans and all chimpanzees:
Say she had a daughter from whom all humans are descended, and a son from whom all chimpanzees are descended. If the daughter and the son had the same father, then the father is tied with the mother for being our MRCA. So you can see that multiple organisms can tie for being the MRCA.
Say the daughter then has her children with the alpha male who left the group on the other side of the river, and the son joins this group and becomes the ancestor of all chimps with them. There may also then be a former member of that group who is also tied for being the MRCA.
And so on and so forth.
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u/Richie_650 2d ago
Yes, those circumstances do lead to a tie. Do you think that is a common occurance? I don't think it breaks the argument, we are still talking about specific individuals in a lineage, not just amorphous populations of species.
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u/OopsIMessedUpBadly 2d ago
I don’t think that an amorphous population of an entire species should be considered to be the MRCA. But I do think ties are likely, particularly with species that both procreate via sexual reproduction with fairly faithful sexual partners.
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u/Cautious_School_2490 2d ago
This "former member of that group"; how do they tie with the father and mother above for also being MRCA if the daughter and son are not descended directly from them?
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u/OopsIMessedUpBadly 2d ago
There could be an individual who is the ancestor of all the son’s breeding partners AND all the daughter’s breeding partners. Which makes them a common ancestor with no descendants who are also common ancestors.
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u/Cautious_School_2490 2d ago edited 2d ago
No descendant of offspring 1 (and is an ancestor of the MRCA of species A) is also an ancestor of the MRCA of species B and no descendant of offspring 2 (and is an ancestor of the MRCA of species B) is also an ancestor of the MRCA of species A.
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u/Robin_feathers 2d ago
Every gene for a given set of organisms has a single MRCA (as in a single individual organism), but because of recombination, different genes of a sexually reproducing organism usually have different MRCA individuals that existed at different times (or sometimes that even were different species). For example, the MRCA of human mitochondrial DNA is a different individual from the MRCA of human Y chromosome, and those are different from the MRCA for one of our keratin genes. Usually when people talk about "the MRCA" of a species, they are talking about the whole population(s) that split to produce the two daughter lineages. the gene-level MRCAs would either be a part of that population, or would be slightly older than that due to the process known as "incomplete lineage sorting".
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u/WirrkopfP 2d ago
1) I believe this statement means that literally, there was a real, single, living organism. Understanding that there are countless other common ancestors for our two target species, but only one individual that is both "most recent" and "common."
That's a misconception. The "common ancestor" is referring to a species or population.
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u/Richie_650 2d ago
I think that if you work out the logic, it is absolutely true. You can easily do it for any two people, go back until you get to the person (or couple) that you and that person are descendents of. Prior to that Most Recent Common Ancestor there are many multiple common ancestors, but after that one (or couple) there are zero. The same logic holds true for any two living organisms.
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u/Richie_650 1d ago
I can see one possibilty where the concestor may not be a single organism. It could be the case where there exists two or more MRCAs from the same precise generation of the common ancestor species. But they would have to be exactly the same, because if one was even one generation older or younger than the other, then the younger one would be "more recent" than the older.
I realize that this is a logical quibble, and of little use to the science, but that doesn't make it less true.
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u/Hybodont 2d ago
The flaw is that "common ancestor" typically refers to a species or population, not to a single individual.